Friction linings such as, for example, brake linings and clutch linings, are usually based on asbestos fibers, mixtures of asbestos fibers and other heat resistant inorganic or organic fibers, asbestos-free mixtures of heat resistant inorganic or organic fibers, or metal powders such as iron powder, copper powders, steel powder or mixtures thereof, in combination with an organic monomeric or polymeric binder or binder mixture. Such friction linings may also contain fillers such as sound- and vibration-damping materials. The binder or binder mixture which is present in the starting mixtures is heat-curable and forms a solid and temperature-stable matrix in its cured state for the various components of the friction lining. While earlier common friction linings were based primarily on asbestos, more and more effort is now being made to replace the asbestos content in friction linings with other materials for the now familiar health reasons, in order to obtain friction linings which are entirely free of asbestos. In addition to the above components, asbestos-containing as well as asbestos-free friction linings usually also contain one or more solid lubricants in order to improve the tribological properties of such materials, particularly their friction and wear properties.
A friction lining based on asbestos fibers is described, for instance, in German Patent No. 3,046,696. A metal-based friction lining is the subject, for instance, of German Patent No. 2,924,540. German Patent No. 2,514,575 is directed to a solid lubricant combination which can also be incorporated in friction linings, among other things. Combination B described in Ex. 2 thereof (45% by wt. graphite, 36% by wt. zinc sulfide, 9% by wt. calcium fluoride and 10% by wt. molybdenum disulfide) is available commercially under the trademark LUBOLID 7365 and is already being added to asbestos-containing and asbestos-free friction linings in order to improve their tribological properties.
The known solid lubricant combinations and the friction linings prepared with them--fiber based as well as metal based--are on the whole not entirely satisfactory with respect to their tribological properties. This is especially true of the above product known as LUBOLID 7365. Friction linings having perhaps satisfactory wear properties and even adequately narrow coefficients of friction can indeed be prepared therewith, but not also friction linings which offer practically vibration-free operation and which transfer practically no lining material to the metallic friction surfaces. In addition, these solid lubricant combinations cannot always be incorporated with equally good results in the various compositions for preparing friction linings, so they cannot be universally incorporated into friction linings based on asbestos fibers, other fibers or even metal powders.
It is therefore the object of this invention to prepare a new solids combination which can be used especially as an additive to friction linings in order to improve their tribological properties, which can be universally incorporated in a great variety of compositions for the preparation of friction linings, especially in asbestos-free and semi-metallic friction linings, and which yields friction linings having low wear properties and narrow coefficients of friction which, above all, offer vibration-free operation, and which do not result in the transfer of lining material to the opposing friction plane of the article in question such as, for example, a brake disk or a clutch plate.
According to the present invention, this problem is now solved by the solids combination and its use as disclosed herein.